1. Field of the Invention
The invention relates to the field of audio peak limiters.
2. Prior Art
The circuit of the present application is an improvement over the distortion-cancellation circuit described in U.S. Pat. No. 4,208,548 (hereinafter the '548 patent). Briefly, this patent describes a method of cancelling clipping-induced distortion containing low-frequency spectral energy by first subtracting the output of the clipper from its input. This differential signal, representing the distortion added by the clipping process, is then applied to a low-pass filter. The output of the low-pass filter is added to the clipped signal, where the clipped signal is delayed by an amount equal to the group delay of the distortion-cancelling low-pass filter.
U.S. Pat. No. 4,495,643 (hereinafter the '643 patent) discusses an improvement of the technique described in the '548 patent that uses Hilbert transforms. The system described in the '643 patent requires the use of relatively expensive circuit elements, including a 90.degree. phase-difference network and a vector sum generator. The '643 patent contains an extensive description of the limitations of the earlier technique set forth in the '548 patent, and provides background that will further the appreciation of the present invention.
As will be seen, the present invention simplifies the circuit of the '643 by eliminating the need for a vector sum generator and for a 90.degree. phase-difference network.
The primary problem with the circuit set forth in the '548 is its handling of low-frequency program material. Bass material processed by the circuit is essentially not peak limited because the clipped waveform is reassembled at its original peak level when the distortion-cancellation signal is summed with the clipped signal. The present invention eliminates this problem by using a circuit conceptually similar to that of the '643 but which is less expensive and which does not require that the audio signal be passed through an allpass filter. (Some experts feel that such filters cause the audio quality to be audibly degraded because different frequencies are delayed by different amounts of time).